Since 1 April 2016 we are only able to provide data in a compressed format, using a library called FPack. The advantage is that it reduces the file size by a factor 3.5 but can still be read (without needing to decompress it) by our image pipeline and most astronomical software (particularly SAO DS9 and APT).
If you cannot use data which has been compressed with FPack you can use the following steps to decompress it back into the conventional FITS format.
If you have a copy of DS9 you can use this to convert your images. Open the image ( File > Open...), then save the image as a regular FITS file ( File > Save as Fits).
If you are using FITS Liberator, you do not need to unpack the FITS files. Just remove the .fz from the filename, leaving .fits on the end.
If you are still having problems with this please use our feedback page for assistance.